Flowmeter with a measuring device implementing a tomographic measuring principle

ABSTRACT

A flowmeter for determining the flow of a multi-phase medium through a measuring tube has a first and a second measuring device, one of which operates on a tomographic measuring principle and one of uses a measuring principle based on nuclear magnetic resonance. The first measuring device operates in a different manner from the second measuring device, e.g., using a measuring device operating on the measuring principle of pre-magnetization contrast measurement and having a pre-magnetization section with a constant magnetic field. The magnetic field has at least one component perpendicular to the direction of flow of the multi-phase medium and is generated by using magnetic field generating elements, which are arranged around the measuring tube. Additionally, an assembly for exciting nuclear spin by a RF excitation pulse or a RF excitation pulse sequence is part of the measuring device.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a flowmeter for determining the flow of amultiphase medium flowing through a measuring tube having a measuringdevice implementing a tomographic measuring principle. The inventionalso relates to a method for operating such a flowmeter.

2. Description of Related Art

The atomic nuclei of the elements that have nuclear spin also have amagnetic moment caused by the nuclear spin. The nuclear spin can beregarded as angular momentum describable by a vector andcorrespondingly, the magnetic moment can also be described by a vector,which is oriented parallel to the vector of the angular momentum. If amacroscopic magnetic field is present, the vector of the magnetic momentof the atomic nucleus tends to orient itself parallel to the vector ofthe macroscopic magnetic field at the atomic nucleus. Here, the vectorof the magnetic moment of the atomic nucleus precesses around the vectorof the macroscopic magnetic field at the atomic nucleus. The frequencyof the precession is called Larmor frequency ω_(L) and is proportionalto the magnitude of the magnetic field strength B. The Larmor frequencyis calculated according to ω_(L)=γ·B where γ is the gyromagnetic ratio,which is at a maximum for hydrogen atoms. The gyromagnetic ratioindicates the proportionality factor between the angular momentum or thespin of a particle and the associated magnetic moment.

Measurement and analysis methods that use the properties of precessionof atomic nuclei having a magnetic moment when a macroscopic magneticfield is present are called nuclear magnetic resonance measurement oranalysis methods. Nuclear magnetic resonance is abbreviated to NMR.

An important representative of the measuring principles is magneticresonance tomography, also called magnetic resonance imaging, MRI.Normally, electric signals induced by the precessing atomic nuclei underdifferent limiting conditions in a sensor coil are used as outputvariable for the measurement and analysis method.

An example of measuring devices that use magnetic resonance are nuclearmagnetic flowmeters, which measure the flow of a multiphase mediumflowing through a measuring tube and analyze the medium.

A requirement for analysis using nuclear magnetic resonance is that thephases of the medium to be analyzed are able to be excited intodistinguishable nuclear magnetic resonances. The analysis can includethe flow velocity of the individual phases of the medium and therelative fractions of the individual phases in the multiphase medium.Nuclear magnetic flowmeters can, for example, be used for analysis ofmultiphase mediums extracted from oil sources. The medium then consistsessentially of the phases crude oil, natural gas and salt water, whereinall phases contain hydrogen atomic nuclei.

The analysis of the medium extracted from oil sources can also takeplace using so-called test separators. These channel off a small portionof the extracted medium, separate the individual phases of the mediumfrom one another and determine the fractions of the individual phases inthe medium. However, test separators are not able to reliably measurecrude oil fractions of less than 5%. Since the crude oil fractions ofmany sources is already less than 5%, it is not possible at this time toeconomically exploit these sources using test separators. In order tofurther economically exploit sources with a very small crude oilfraction, accordingly exact flowmeters are necessary.

Normally, electric signals induced by the precessing atomic nuclei afterexcitation in a sensor coil are used as output variable for evaluation.A requirement for the measurement of a multi-phase medium is, as alreadymentioned, that the individual phases of the medium can be excited todistinguishable nuclear magnetic resonances. The magnitude of theelectric signals induced by the precessing atomic nuclei of one phase ofthe medium in the sensor coil is dependent on the number of precessingatomic nuclei per volume element in this phase, thus depending on thedensity of the phase, but also on the influence time of the precessingatomic nuclei in the influencing, controlled magnetic field. Thus, themagnitude of the induced electric signal in the liquid phases is greaterthan in the gaseous phases.

Spatial information necessary for magnetic resonance imaging is, forexample, applied to the sample with a gradient field. Since the Larmorfrequency of the atomic spin is proportional to the magnetic fieldstrength, a location-dependent distribution of different Larmorfrequencies of the atomic spins is created by the gradient field andthus a spatial dependency of the electric signals induced by the atomicnuclei.

As described above, the MRI signal is dependent on the density of themedium. In a comparison of the average values of the signal amplitudesper cubic meter of gas, oil and water, it can be determined that thesignal from gas is clearly different than that of oil and water,however, there is almost no difference between the signals from oil andwater. The strength of the signal can be expressed by the so-calledhydrogen index HI. The hydrogen index HI describes the relative fractionof hydrogen atoms of a medium compared to water. Accordingly, thehydrogen index of water HI_(Water)=1. The indices for oil and gas areHI_(oil)=0.9-1.1 and HI_(gas)=0-0.2. With the help of the MR signals, itis easy to distinguish gas, on the one hand, and liquid (consisting ofwater and oil), on the other hand. Differentiating between water and oilis difficult or very complex, since the amplitudes of the MR signals arebarely different.

As already described, nuclear magnetic measurement and analysis methodsare based on the effect that the magnetic moments of the nucleus arealigned along the field line of an externally applied magnetic field.This leads to a bulk magnetization of the medium. The rate at which thismagnetization establishes is determined by the so-called spin latticerelaxation time T₁ and has an exponential course.

A further measurement variable typical for nuclear magnetic measurementand analysis methods is the spin-spin relaxation time T₂. This time is ameasure for inhomogeneity in the magnetic field surrounding the onesingle spin.

The mechanisms, which determine the values for T₁ and T₂, are dependenton the molecular dynamics of the test sample. The molecular dynamicsare, in turn, dependent on the size of the molecules and also on theintermolecular spacing. These are different for each medium.Accordingly, different mediums also have different values for T₁ and T₂.

A measurement method known from the prior art for characterizingindividual phases of a multi-phase medium is given by the measuringprinciple of pre-magnetization contrast measurement. This measuringprinciple is based on the difference in the T₁ time for different phasesof a multiphase medium and is suitable in a distinct manner fordetermining the oil fraction and the water fraction as well as therelative ratio of the oil fraction to the water fraction in a sample.

The multiphase medium flows through a section interfused with a constantmagnetic field. Here, the magnetic field has at least one componentperpendicular to the direction of flow of the medium. Since thealignment of the magnetic moments in the magnetic field is dependent onthe respective phase of the medium, different formation of magnetizationin the individual phases results at the same exposure time. The exposuretime of the magnetic field is determined by the length of the sectioninterfused by the constant magnetic field and the flow velocity of themedium.

In general, the longitudinal relaxation time T₁ of oil is much smallerthan that of water. Accordingly, the magnetization of oil parallel tothe outer magnetic field establishes more quickly than for water. Byvarying the length of the pre-magnetization section, the signals fromoil and water are each formed at a different level, so that the ratio ofoil fraction to water fraction in the medium can be determined from theoil-water signal ratio dependent on the pre-magnetization section. Thestrong contrast between oil signal and water signal depending on thepre-magnetization section offers a good possibility for determining theoil to water ratio (OWR) of the medium.

Since the signal of the gas fraction is very weak, the method is, on theone hand, independent of the gas fraction. However, on the other hand,it is not suitable for determining the gas fraction, so that not allthree phases of the medium can be characterized using the measuringprinciple of pre-magnetization contrast measurement.

Another measuring principle, which is also often used in flowmeasurement technology and is not based on nuclear spin resonance is byelectrical capacitance tomography (ECT).

Electrical capacitance tomography is a method known from the prior artfor measuring and characterizing multiphase media. It is generallysuitable for dielectric materials and is based on the fact thatdifferent materials have different permittivities.

A typical measuring device for electrical capacitance tomography isdesigned in such a manner that a certain number of electrodes arearranged around a measuring tube. Measuring devices known from the priorart usually have eight, twelve or sixteen electrodes.

In a measuring device of the type being described, an excitation voltageis applied to an electrode and the induced voltage/the current ismeasured in all other electrodes, while their electric potential is keptat zero. This is carried out for all existing electrodes. Using theexample of a measuring device with eight electrodes, the first electrodeis used in a first step as excitation electrode and the second to eighthelectrodes are used as detector electrodes. In the next step, the secondelectrode is used as an excitation electrode and the third througheighth electrodes are used as detector electrodes, etc. In a measuringdevice with N electrodes, there are N·(N−1)/2 electrode paircombinations, and thus, N·(N−1)/2 measuring values of capacity fromwhich an image can be constructed. The construction occurs by means ofan evaluation algorithm, which is not explained in detail here.

Since the capacity is dependent on the permittivity, i.e., thepermeability of a material for an electric field, of the multiphasemedium between the electrodes, it is thus possible to dissolve thedistribution of the individual phases using the measured values, sinceeach phase of the medium has a different permittivity.

The permittivity of gas is about 1, ∈_(r)≈1, the permittivity of oilbetween 2 and 4, ∈_(r)≈2-4, and the permittivity of water is greaterthan 50, ∈_(r)>50. Using the values shown here for the permittivity ofthe individual phases, it can be observed that it is very difficult andcomplex to separate the gaseous phase from the oil phase, since thevalues of permittivity characterizing the two phases are not far fromone another, namely almost the same. Electrical capacitance tomographywas shown above to be a good method for determining the hydrocarbonfraction of a multi-phase medium, which is made up of the oil fractionand the gas fraction, and the water fraction of the medium.

The measuring principles described above, as shown, have greatadvantages in the measurement of certain properties of a multiphasemedium. On the other hand, however, they also have the showndisadvantages or limitations so that the determination of all threephases of the multiphase medium is either not possible, inexact orextremely complex.

SUMMARY OF THE INVENTION

The object of the present invention is thus to provide a flowmeter and amethod for operating the flowmeter according to the invention in whichall three phases of the multiphase medium can be reliably determined ina simple manner.

The flowmeter according to the invention in which the above describedobject is met, is initially and essentially characterized in that atleast one further measuring device is provided and at least one of themeasuring devices implements a measuring principle based on nuclearmagnetic resonance. The second measuring device can realize either alsoa tomographic or even a non-tomographic measuring principle.

An advantage of the flowmeter according to the invention exists, ascompared to the flowmeters known from the prior art, in that it ispossible to determine all three phases of the multiphase medium withouthaving to separate the individual phases. As a result, the requiredeffort for the determination of the flow of a multiphase medium flowingthrough a measuring tube is notably reduced.

Different measuring principles have different advantages anddisadvantages. In combining of two measuring device in one flowmeter,which implement the different measuring principles, the disadvantages ofone measuring principle can be at least partially compensated by theadvantages of the other measuring principle, so that optimizedmeasurement results can be achieved with the combined measuringprinciples.

A preferred design of the flowmeter according to the invention, in whichthe tomographic measuring principle is realized by magnetic resonancetomography, is additionally characterized in that one measuring deviceimplements the measuring principle of electrical capacitance tomography.Here, both measuring devices are arranged consecutively in the directionof flow around the measuring tube with multiphase medium flowing throughit. The first measuring device can be the one that implements thetomographic measuring principle by magnetic resonance tomography, whilethe second measuring device can be the one that implements the measuringprinciple by electrical capacitance tomography. A reversed order of thetwo measuring devices is, however, just as easily possible.

The measuring device realizing the tomographic measuring principle bymagnetic resonance tomography consists of a magnetic resonancetomograph, which is arranged around the measuring tube. The magneticresonance tomograph preferably includes at least one magnetic fieldgenerator for a constant magnetic field and one magnetic field generatorfor a gradient magnetic field, which can be realized preferably by agradient coil. The gradient field can be superimposed over the constantmagnetic field in order to apply position information to the sample.Additionally, the magnetic resonance tompgraph preferably also includesa signal coil for generating a RF excitation pulse or a RF pulsesequence for exciting the nuclear spin as well as a detector coil inorder to be able to detect the measuring signal generated by the nuclearspin. Here, the signal coil and the detector coil can be realized eitheras different coils or as one coil.

Without limiting the generality, the flow direction of the mediumthrough the measuring tube is defined as x-direction. By introducing acoordinate system, the x-axis is chosen along the longitudinal axis ofthe measuring tube. The y-axis is defined as the horizontal axis, thez-axis is defined as the vertical axis.

The magnetic resonance tomograph is now designed in such a manner thatspatial information necessary for tomography can be applied in differentdirections. The spatial information can, for example, be generated by agradient field of different gradient directions. This is advantageous,in particular, in that the measurement of each character of the mediumcan be adapted, which allows for a reduction of the measuring effort.

Generating a gradient field along the z-direction is, in particular,suitable when a medium is present in which the liquid phase and the gasphase are “separate”, i.e., for example, the liquid phase flows in thelower region of the tube and the gas phase, due to the lower density,flows in the upper region of the tube. For such a medium, it issufficient to apply spatial information only in the z-direction in orderto completely characterize the medium.

If the multiphase medium is such that the gas phase is mixed with theliquid phase over the entire cross-section of the measuring tube, i.e.,for example, in the form of gas bubbles in the liquid, spatialinformation only in the z-direction is not sufficient for characterizingthe entire medium. Moreover, it is now necessary to be able to measureeach point along the cross-section of the measuring tube. Consequently,it is necessary to apply spatial information both in the z-direction aswell as the y-direction. Implementation can be carried out in twodifferent manners. On the one hand, a gradient field can be generatedalong the z-axis, G=G_(z)·e_(z). Directly thereafter, a gradient fieldalong the y-axis can be generated, G=G_(y)·e_(y). In this manner, thespatial information is generated and measured along the z-axis in afirst step and in a second step, the position information is generatedand measured along the y-axis. A combination of the measuring resultsets generated in this manner results in a complete image. On the otherhand, it is possible to generate a gradient field that has both agradient along the z-axis as well as a gradient along the y-axis,G=G_(y)·e_(y)+G_(z)·e_(z). Spatial information is applied directly tothe entire cross-section of the measuring tube by such a field.

It is now possible, to encode the position information in differentmanners with the help of the gradient field.

If the gradient field is already applied before the excitation of thespin system by the excitation pulse, i.e., if the spins are alreadyprecessing depending on position at different Larmor frequencies beforeexcitation, a certain part of the spins can be chosen by the pulse widthof the excitation pulse, which is then excited. Thus, a selectiveexcitation of the spin system is present and consequently only theselective excited part of the spins emits a measuring signal.

It is also possible to encode the spatial information using a phaseshift of the spin. The gradient field is applied between the excitationof the spin system by an excitation pulse and the reading out of thesignals generated by the spin system for a certain time interval. Theprecession frequency is changed depending on the position by thegradient field since, as already mentioned, the Larmor frequency isproportional to the magnetic field strength. If the gradient field isturned off again, the spins precess again at their “old” frequency,however there was a position-dependent change of the phase of theexcited spin, this is called phase encoding.

If the spins are excited by an excitation pulse and then a gradientfield is applied during reading (“reading gradient field”), this leadsto the spin emitting signals with different, position-dependentfrequencies during measurement. The measured “frequency mixture” can bedecoded using a Fourier transformation. An encoding of spatialinformation is thus also possible via the frequency, this is calledfrequency encoding.

The electrical capacitance tomograph provided as further measuringdevice can be designed in manner as is described above in general forelectrical capacitance tomographs, so that it is not necessary to gointo detail here.

The electrical capacitance tomograph includes a number of electrodes,which are symmetrically arranged around the measuring tube. The numberof electrodes can be arbitrarily chosen. The electrodes are preferablyattachable to the outside of the measuring tube. This guarantees thatthe flow of the medium is not disturbed and influenced by theelectrodes. An excitation voltage is applied to one of the electrodes bya measuring unit and the measuring signal of the remaining electrodes isdetected. This procedure is repeated as described above for allelectrodes and the distribution of permittivities in the samplereconstructed using an algorithm.

Another preferred embodiment of the flowmeter according to the inventionin which the tomographic measuring principle is realized by electricalcapacitance tomography, is additionally characterized in that onemeasuring device implements the measuring principle of pre-magnetizationcontrast measurement. The measuring device implementing the measuringprinciple of pre-magnetization contrast measurement includes apre-magnetization section interfused with a constant magneticfield—wherein the magnetic field has at least one componentperpendicular to the flowing medium, as well as a unit, with which thenuclear spins can be excited by a RF excitation pulse or a RF excitationpulse sequence and the measuring signal generated by the nuclear spincan be detected.

In order to implement the measuring principle of the pre-magnetizationcontrast measurement, the pre-magnetization section interfused with theconstant magnetic field must be variable in length, which can berealized in different manners.

It is described above, that the invention also relates to a method foroperating a flowmeter for determining the flow of a multiphase mediumflowing through a measuring tube, wherein a measuring deviceimplementing a tomographic measuring principle is part of the flowmeter.

There are plural possibilities here, wherein it is common to allpossibilities that two measuring principles are used and at least one ofthe measuring principles is a measuring principle based on nuclear spinresonance.

If the tomographic measuring principle is realized by magnetic resonancetomography, then in addition, either the measuring principle ofelectrical capacitance tomography or the measuring principle ofpre-magnetization contrast measurement can be implemented. If thetomographic measuring principle is realized by electrical capacitancetomography, then in addition the measuring principle ofpre-magnetization contrast measurement can be implemented.

If the tomographic measuring principle is realized by magnetic resonancetomography and also the measuring principle of electrical capacitancetomography is implemented, it can be proceeded such that the gaseousfraction α_(G) and the liquid fraction α_(L), being the sum of the waterfraction α_(W) and the oil fraction α_(O), α_(L)=α_(W)+α_(O), aremeasured by means of magnetic resonance tomography, where the spatialinformation is encoded by selective excitation and/or phase encodingand/or frequency encoding and a gradient magnetic field is applied alongthe z-axis, G=G_(z)·e_(z), or a gradient magnetic field is applied alongthe y-axis, G=G_(y)·e_(y), or a gradient magnetic field is first appliedalong the z-axis and then along the y-axis and the data records arecombined, or a gradient magnetic field is applied simultaneously alongthe z-axis and the y-axis, G=G_(y)·e_(y)+G_(z)·e_(z), that the waterfraction α_(W) and the hydrocarbon fraction α_(C), being the sum of theoil fraction α_(O) and the gaseous fraction α_(G), α_(C)=α_(O)+α_(G),are measured by means of electrical capacitance tomography and the waterfraction α_(W) and the hydrocarbon fraction α_(C) are determined by thedistribution of the permittivities or by the distribution of theconductivity of the medium and that the oil fraction α_(O) is calculatedby subtracting the water fraction α_(W) measured by means of electricalcapacitance tomography from the liquid fraction α_(L) measured by meansof magnetic resonance tomography, which is α_(O)=α_(L,MR)−α_(W,ECT), orthat the oil fraction α_(O) is calculated by subtracting the gaseousfraction α_(G) measured by means of magnetic resonance tomography fromthe hydrocarbon fraction α_(C) measured by means of electricalcapacitance tomography, which is α_(O)=α_(C,ECT)−α_(G,MR).

Using the method described above, it is recommended further to determinethe mean conductivity of the medium from the measured values by means ofelectrical capacitance tomography, to determine the additional load tothe RF resonator circuit of the magnetic resonance tomograph due to themean conductivity of the medium and/or at least of one conducting phaseof the multiphase medium and to enhance the RF-power fed in the mediumfor exciting the nuclear spins, such that the influence of theadditional load due to the mean conductivity on the excitation of thenuclear spins is compensated.

Using the method described above, it is further recommended to proceedsuch that a conductivity map is generated over the cross sectional areaof the measuring tube by means of electrical capacitance tomography,that the mean conductivity of the medium is calculated from theconductivity map, and that additionally the local deviations of theconductivity from the mean conductivity of the medium are determinedwith the conductivity map, that the additional load to the RF resonatorcircuit of the magnetic resonance tomograph caused by the meanconductivity of the medium is determined, and that additionally thelocal dampings of the RF field due to the local deviations of theconductivities from the mean conductivity are determined, and that theRF power fed in the medium for exciting the nuclear spins is enhancedsuch that the influence of the additional load caused by the meanconductivity on the excitation of the nuclear spins is compensated andadditionally RF power is fed locally in the medium, such that theinfluence of the local conductivities deviating from the meanconductivity on the excitation of the nuclear spins is compensated.

If magnetic resonance tomography is realized as the tomographicmeasuring principle using the method according to the invention, andadditionally the measuring principle of pre-magnetization contrastmeasurement is implemented, a further teaching of the invention ischaracterized in that the oil fraction α_(O) and the water fractionα_(W) are measured by means of pre-magnetization contrast measurement,wherein the pre-magnetization contrast is realized by changing thelength of the pre-magnetization section or by varying the measuringpositions and that the gaseous fraction α_(G) is measured by means ofmagnetic resonance tomography, where the spatial information is encodedby selective excitation and/or phase encoding and/or frequency encodingand a gradient magnetic field is applied along the z-axis,G=G_(Z)·e_(z), or a gradient magnetic field is applied along the y-axis,G=G_(y)·e_(y), or a gradient magnetic field is first applied along thez-axis and then along the y-axis and the measuring result sets arecombined, or a gradient magnetic field is applied simultaneously alongthe z-axis and the y-axis, G=G_(y)·e_(y)+G_(z)·e_(z).

Another realization of the method according to the invention, where thetomographic measuring principle is realized by magnetic resonancetomography and in addition the measuring principle of pre-magnetizationcontrast measurement is implemented, is characterized in that the ratioof the oil fraction α_(O) to the water fraction α_(W) is determined bymeans of pre-magnetization contrast measurement (OWR=α_(O)/α_(W)),wherein the pre-magnetization contrast is realized by changing thelength of the pre-magnetization section or by varying the measuringpositions, that the liquid fraction α_(L) and the gaseous fraction α_(G)are measured by means of magnetic resonance tomography, where thespatial information is encoded by selective excitation and/or phaseencoding and/or frequency encoding and a gradient magnetic field isapplied along the z-axis, G=G_(z)·e_(z), or a gradient magnetic field isapplied along the y-axis, G=G_(y)·e_(y), or a gradient magnetic field isfirst applied along the z-axis and then along the y-axis and themeasuring result sets are combined, or a gradient magnetic field isapplied simultaneously along the z-axis and the y-axis,G=G_(y)·e_(y)+G_(z)·e_(z) and that the water fraction α_(W) iscalculated from the liquid fraction α_(L) measured by means of magneticresonance tomography and the ratio of the oil fraction α_(O) to thewater fraction α_(W) OWR by means of measuring the pre-magnetizationcontrast by α_(W)=α_(L,MR)/(OWR+1).

As explained, the method according to the invention may also deal withthe tomographic measuring principle being realized by electricalcapacitance tomography and also using the measuring principle ofpre-magnetization measurement. In detail it can be proceeded such thatthe water fraction α_(W) and the hydrocarbon fraction α_(C), being thesum of the oil fraction α_(O) and the gaseous fraction α_(G),α_(C)=α_(O)+α_(G), are measured by means of electrical capacitancetomography and the water fraction and the hydrocarbon fraction aredetermined by the distribution of the permittivities or by thedistribution of the conductivity of the medium, that the oil fractionα_(O) and the water fraction α_(W) are measured by means of measuringthe pre-magnetization contrast, wherein the pre-magnetization contrastis realized by changing the length of the pre-magnetization section orby varying the measuring positions and that the gaseous fraction α_(G)is calculated by subtracting the oil fraction α_(O) measured by means ofpre-magnetization contrast measurement from the hydrocarbon fractionα_(C) measured by electrical capacitance tomography,α_(G)=α_(C,ECT)−α_(O,VM).

It is also possible to proceed such that the water fraction α_(W) andthe hydrocarbon fraction α_(C), being the sum of the oil fraction α_(O)and the gaseous fraction α_(G), α_(C)=α_(O)+α₃, are measured by means ofelectrical capacitance tomography and the water fraction and thehydrocarbon fraction are determined by the distribution of thepermittivities or by the distribution of the conductivity of the medium,that the ratio of the oil fraction α_(O) to the water fraction α_(W) isdetermined by means of pre-magnetization contrast measurement(OWR=α_(O)/α_(W)), wherein the pre-magnetization contrast is realized bychanging the length of the pre-magnetization section or by varying themeasuring positions and that first the oil fraction α_(O) is determinedfrom the measured values by multiplying the water fraction α_(W)measured by means of electrical capacitance tomography with the OWRdetermined by means of pre-magnetization contrast measurement,α_(O)=OWR·α_(W,ECT), and then the gaseous fraction α_(G) is determinedby subtracting the calculated oil fraction from the hydrocarbon fractionα_(C) measured by means of electrical capacitance tomography,α_(G)=α_(C,ECT) α_(O).

In the particular, realization of the method as described above, wherethe tomographic measuring principle is realized by electricalcapacitance tomography and additionally the measuring principle ofpre-magnetization measurement is implemented, it can supplementary beproceeded such that the mean conductivity of the medium is determinedwith the values measured by means of electrical capacitance tomography,that the additional load to the RF resonator circuit of the magneticresonance tomograph due to the mean conductivity of the medium and/or atleast one conducting phase of a multiphase medium is determined, andthat the RF power fed in the medium for exciting the nuclear spins isenhanced such that the influence of the additional load due to the meanconductivity on the excitation of the nuclear spins is compensated.

In special this can supplementary be realized such that a conductivitymap is generated over the cross sectional area of the measuring tube bymeans of electrical capacitance tomography, that the mean conductivityof the medium is calculated from the conductivity map, and thatadditionally the local deviations of the conductivity from the meanconductivity of the medium are determined with the conductivity map,that the additional load to the RF resonator circuit of the magneticresonance tomograph due to the mean conductivity of the medium isdetermined, and that additionally the local damping of the RF field dueto the local deviations of the conductivities from the mean conductivityare determined, and that the RF power fed in the medium for exciting thenuclear spins is enhanced such that the influence of the additional loaddue to the mean conductivity on the excitation of the nuclear spins iscompensated and additionally RF power is fed locally in the medium, suchthat the influence of the local conductivities deviating from the meanconductivity on the excitation of the nuclear spins is compensated.

Finally, the method according to the invention, as it is describedabove, can be used for determining the salinity of the medium and/or atleast one conducting phase of a multiphase medium by the conductivity ofthe medium and/or of at least one conducting phase of the multiphasemedium.

In detail there are various possibilities for designing and furtherdeveloping the flowmeter according to the invention. Here, reference ismade to the following detailed description in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a first embodiment of a flowmeter according to theinvention implementing the measuring principle of magnetic resonancetomography and the measuring principle of electrical capacitancetomography,

FIG. 2 shows a second embodiment of a flowemeter according to theinvention implementing the measuring principle of electrical capacitancetomography and the measuring principle of pre-magnetization measurementand

FIG. 3 shows a third embodiment of a flowmeter according to theinvention implementing the measuring principle of magnetic resonancetomography and the measuring principle of pre-magnetization contrastmeasurement.

DETAILED DESCRIPTION OF THE INVENTION

All figures show a flowmeter 1 for determining the flow of a multiphasemedium flowing through a measuring tube 2. All flowmeters 1 shown in thefigures have in common, first of all, a first measuring device 3 and asecond measuring device 4. At least one of the first and secondmeasuring devices 3, 4 implements a tomographic measuring principle.

In the embodiment of the flowmeter 1 according to the invention shown inFIG. 1, the first measuring device 1 is realized by an electricalcapacitance tomograph. This electrical capacitance tomograph 5 has anumber of electrodes 6 symmetrically arranged around the measuring tube2. The electrical capacitance tomograph 5 is designed such that theelectrodes are attachable to the outside to the measuring tube 2. Thisguarantees that the flow of the multiphase medium through the measuringtube 2 is not influenced or disturbed by the electrodes 6. The measuringsignals arising at the electrodes 6 are evaluated by an evaluation unit(not shown here) and are constructed to a two dimensional permittivitydistribution map of the cross-section area of the measuring tube 2 by asuitable algorithm.

The second measuring device 4 of the flowmeter 1 shown in FIG. 1 isrealized by a magnetic resonance tomograph 7, which is also arrangedaround the measuring tube 2. The magnetic resonance tomograph 7 includesa unit for generating a constant magnetic field, (not shown), and also aunit for generating a gradient magnetic field G (not shown), that can besuperimposed on the constant magnetic field, a exciting coil forgenerating a RF exciting pulse or a RF exciting pulse sequence (notshown) and also a detection coil (not shown), with which the measuringsignal generated by the nuclear spins can be detected. The exciting coiland the detecting coil can be realized a single coil. It is possible togenerate a gradient field g along the z-direction, G=G_(z)·e_(z) and/ora gradient field along the y-direction G=G_(y)·e_(y). This can be donesimultaneously, G=G_(y)·e_(y)+G_(z)·e_(z), as well as one after theother. The definition of the x-, y- and z-directions is indicated in thedrawings below the FIG. 3 legend.

For the embodiment according to FIG. 2, the first measuring device 3 isrealized by a measuring device implementing the measuring principle ofpre-magnetization contrast measurement, which shows a pre-magnetizationsection 8 with a constant magnetic field. The magnetic field has atleast one component perpendicular to the direction of flow of themultiphase medium and is generated by magnetic field generating elements9, which are arranged around the measuring tube 2. The section permeatedby the magnetic field depends on the number of magnetic field generatingelements 9 and the direction of the generated magnetic fields withrespect to each other.

Also an assembly 10 for exciting the nuclear spins by a RF excitingpulse or a RF exciting pulse sequence and for measuring the measuringsignals generated by the nuclear spins is part of the first measuringdevice 3.

According to the embodiment of a flowmeter according to the inventionshown in FIG. 4, the second measuring device 4 is realized by anelectrical capacitance tomograph 5. This electrical capacitancetomograph 5 can be realized in the same manner and the same things canbe achieved as was already mentioned in conjunction with the embodimentaccording to FIG. 1.

For the embodiment according to the invention shown in FIG. 3, what wasalready explained above is valid in that it contains a first measuringdevice 3 and a second measuring device 4. Here, the first measuringdevice 3 implements the measuring principle of pre-magnetizationmeasurement and contains a pre-magnetization section 8, which ispermeated by a constant magnetic field. Also here, the magnetic field isgenerated by a number of magnetic field generating elements 9, which arearranged around the measuring tube 2, and has at least one componentperpendicular to the direction of flow of the multiphase medium. Alsohere, the measuring device 3 contains an assembly 10 for exciting thenuclear spins by a RF exciting pulse or a RF exciting pulse sequence andfor measuring the measuring signals generated by the nuclear spins. Thepre-magnetization 8 interfused by the effective magnetic field isdefined and varied by the number of the magnetic field generatingelements 9 and/or the direction of the magnetic fields generated by themagnetic field generating elements 9 with respect to each other.

For the schematically shown embodiment shown in FIG. 3, it is furthervalid that the tomographic measuring principle is realized by magneticresonance tomography. Therefore, the embodiment contains a magneticresonance tomograph 7. This magnetic resonance tomograph 7 can berealized in the same manner and the same results can be reached as wasalready mentioned in conjunction with the magnetic resonance tomograph 7belonging to the embodiment according to FIG. 1.

What is claimed is:
 1. Flowmeter for determining the flow of amultiphase medium flowing through a measuring tube, comprising: atomographic measuring device and at least one further measuring device,wherein at least one of the measuring devices comprises a nuclearmagnetic resonance measuring device.
 2. Flowmeter according to claim 1,wherein the tomographic measuring device comprises a magnetic resonancetomography device, and wherein another of said measuring devices is anelectrical capacitance tomography device.
 3. Flowmeter according toclaim 1, wherein the tomographic measuring device comprises a electricalcapacitance tomography device, and wherein another of said measuringdevices comprises a pre-magnetization contrast measurement device. 4.Flowmeter according to claim 1, wherein the tomographic device comprisesa magnetic resonance tomography device, and wherein another of saidmeasuring devices comprises a pre-magnetization contrast measurementdevice.
 5. Method for operating a flowmeter for determining the flow ofa multiphase medium flowing through a measuring tube having a measuringdevice implementing a tomographic measuring principle, wherein twomeasuring principles are exerted and at least one of the measuringprinciples is based on nuclear magnetic resonance.
 6. Method accordingto claim 5, wherein the different tomographic measuring principle iscomprises measuring flow using electrical capacitance tomography. 7.Method according to claim 5, wherein the tomographic measuring principleis realized by magnetic resonance tomography, and wherein the differentmeasuring principle comprises pre-magnetization contrast measurement. 8.Method according to claim 5, wherein the tomographic measuring principleis realized by electrical capacitance tomography, wherein the differentmeasuring principle comprises pre-magnetization contrast measurement isimplemented.
 9. Method according to claim 6, wherein a gaseous fractionαG and a liquid fraction α_(L), being the sum of the water fractionα_(W) and the oil fraction α_(O), α_(L)=α_(W)+α_(O), are measured bymeans of magnetic resonance tomography, where spatial information isencoded by at least one of selective excitation, phase encoding andfrequency encoding and a gradient magnetic field is applied one of alongthe z-axis, the y-axis, first along the z-axis and then along the y-axiswith the measuring result sets being combined and simultaneously alongthe z-axis and the y-axis, wherein the water fraction α_(W) and ahydrocarbon fraction α_(G), being the sum of the oil fraction α_(O) andthe gaseous fraction α_(G), are measured by means of electricalcapacitance tomography and the water fraction and the hydrocarbonfraction are determined by the distribution of one of permittivities andconductivity of the medium and wherein the oil fraction α_(O) iscalculated by subtracting the water fraction α_(W) measured by means ofelectrical capacitance tomography from the liquid fraction α_(L)measured by means of magnetic resonance tomography.
 10. Method accordingto claim 6, wherein a gaseous fraction αG and a liquid fraction α_(L),being the sum of the water fraction α_(W) and the oil fraction α_(O),α_(L)=α_(W)+α_(O), are measured by means of magnetic resonancetomography, where spatial information is encoded by at least one ofselective excitation, phase encoding and frequency encoding and agradient magnetic field is applied one of along the z-axis, the y-axis,first along the z-axis and then along the y-axis with the measuringresult sets being combined and simultaneously along the z-axis and they-axis, wherein the water fraction α_(W) and a hydrocarbon fractionα_(G), being the sum of the oil fraction α_(O) and the gaseous fractionα_(G) are measured by means of electrical capacitance tomography and thewater fraction and the hydrocarbon fraction are determined by thedistribution of one of permittivities and conductivity of the medium andwherein the oil fraction α_(O) is calculated by subtracting the gaseousfraction α_(G) measured by means of magnetic resonance tomography fromthe hydrocarbon fraction α_(C) measured by means of electricalcapacitance tomography.
 11. Method according to claim 10, wherein a meanconductivity of the medium is determined from the measured values bymeans of electrical capacitance tomography, wherein an additional loadapplied to a RF resonator circuit of the magnetic resonance tomographcaused by at least one of a mean conductivity of the medium and at leastof one conducting phase of the multiphase medium is determined andwherein the RF-power fed in the multiphase medium for exciting thenuclear spins is enhanced, such that the influence of the additionalload caused by the mean conductivity on the excitation of the nuclearspins is compensated.
 12. Method according to claim 11, wherein aconductivity map of the medium is generated over a cross-section area ofthe measuring tube by means of electrical capacitance tomography,wherein the mean conductivity of the medium is calculated from theconductivity map, and additionally, local deviations of the conductivityfrom the mean conductivity of the medium are determined with theconductivity map, wherein an additional load to the RF resonator circuitof the magnetic resonance tomograph caused by the mean conductivity ofthe medium is determined, and wherein local damping of the RF field dueto the local deviations of the conductivities from the mean conductivityare determined, and wherein the RF power fed into the medium forexciting the nuclear spins is enhanced such that the influence of theadditional load caused by the mean conductivity on the excitation of thenuclear spins is compensated, and additionally, RF power is fed locallyinto the medium, such that the influence of the local conductivitiesdeviating from the mean conductivity on the excitation of the nuclearspins is compensated.
 13. Method according to claim 7, wherein an oilfraction α_(O) and the water fraction α_(W) of the multiphase medium aremeasured by means of pre-magnetization contrast measurement, wherein thepre-magnetization contrast is realized by changing the length of thepre-magnetization section or by varying the measuring positions andwherein the gaseous fraction α_(G) is measured by means of magneticresonance tomography, spatial information being encoded by at least oneof selective excitation, phase encoding and frequency encoding, andwherein a gradient magnetic field is applied one of along a z-axis,along a y-axis, first along the z-axis and then along the y-axis, andsimultaneously along the z-axis and the y-axis with measuring resultsets being combined.
 14. Method according to claim 7, wherein a ratio ofan oil fraction α_(O) to a water fraction α_(W) of the multiphase mediumis determined by means of pre-magnetization contrast measurement inwhich a length of the pre-magnetization section or measuring positionsare varied, wherein the liquid fraction α_(L) and the gaseous fractionα_(G) are measured by means of magnetic resonance tomography withspatial information being encoded by at least one of selectiveexcitation, phase encoding, and frequency encoding, wherein a gradientmagnetic field is applied one of along a z-axis, along a y-axis, firstalong the z-axis and then along the y-axis with measuring result setsbeing combined, and simultaneously along the z-axis and the y-axis, andwherein the water fraction α_(W) is calculated from the liquid fractionα_(L) measured by means of magnetic resonance tomography and a ratio OWRof the oil fraction α_(O) to the water fraction α_(W) measured by meansof measuring the pre-magnetization contrast in accordance with theequation α_(W)=α_(L,MR)/(OWR+1).
 15. Method according to claim 8,wherein a hydrocarbon fraction α_(C) of the multiphase medium is the sumα_(O)+α_(G) of an oil fraction α_(O) and a gaseous fraction α_(G),wherein the hydrocarbon fraction α_(C) and a water fraction α_(W) aremeasured by means of electrical capacitance tomography and the waterfraction α_(W) and the hydrocarbon fraction α_(C) are determined by adistribution of one of permittivities and conductivity of the medium,wherein the oil fraction α_(O) and the water fraction α_(W) are measuredby measuring pre-magnetization contrast, the pre-magnetization contrastbeing realized by changing one of a length of the pre-magnetizationsection and measuring positions and wherein the gaseous fraction α_(G)is calculated by subtracting the oil fraction α_(O) measured by thepre-magnetization contrast measurement from the hydrocarbon fractionα_(C) measured by electrical capacitance tomography.
 16. Methodaccording to claim 8, wherein a hydrocarbon fraction α_(C) of themultiphase medium is the sum α_(O)+α_(G) of an oil fraction α_(O) and agaseous fraction α_(G), wherein the hydrocarbon fraction α_(C) and awater fraction α_(W) are measured by means of electrical capacitancetomography and the water fraction α_(W) and the hydrocarbon fractionα_(C) are determined by a distribution of one of permittivities andconductivity of the medium, wherein a ratio of the oil fraction α_(O) tothe water fraction α_(W) is determined by measuring pre-magnetizationcontrast realized by changing at least one of the length of apre-magnetization section and measuring positions and wherein first theoil fraction α_(O) is determined from the measured values by multiplyingthe water fraction α_(W) measured by means of electrical capacitancetomography with the said ratio determined by means of pre-magnetizationcontrast measurement, and then the gaseous fraction α_(G) is determinedby subtracting the determined oil fraction from the hydrocarbon fractionα_(C) measured by electrical capacitance tomography.
 17. Methodaccording to claim 16, wherein a mean conductivity of the medium isdetermined from the measured values by means of electrical capacitancetomography, wherein an additional load to the RF resonator circuit ofthe magnetic resonance tomograph caused by at least one of the meanconductivity of the medium and at least one conducting phase of themultiphase medium is determined and wherein RF-power fed into the mediumfor exciting the nuclear spins is enhanced, such that the influence ofthe additional load caused by the mean conductivity on the excitation ofthe nuclear spins is compensated.
 18. Method according to claim 16,wherein a conductivity map of the medium is generated over thecross-sectional area of the measuring tube by electrical capacitancetomography, a mean conductivity of the medium being calculated from theconductivity map, and additionally local deviations of the conductivityfrom the mean conductivity of the medium are determined with theconductivity map, wherein an additional load to the RF resonator circuitof the magnetic resonance tomograph caused by the mean conductivity ofthe medium is determined, and additionally local damping of the RF fielddue to the local deviations of the conductivities from the meanconductivity are determined, and wherein RF power fed into the mediumfor exciting the nuclear spins is enhanced such that the influence ofthe additional load caused by the mean conductivity on the excitation ofthe nuclear spins is compensated and additionally RF power is fedlocally into the medium such that the influence of the localconductivities deviating from the mean conductivity on the excitation ofthe nuclear spins is compensated.
 19. Method according to claim 17,wherein the salinity of at least one of the multiphase medium and atleast one conducting phase of the multiphase medium is determined by theconductivity thereof.
 20. Method according to claim 11, wherein thesalinity of at least one of the multiphase medium and at least oneconducting phase of the multiphase medium is determined by theconductivity thereof.